Positive electrode active materials are used for positive electrodes of secondary batteries such as non-aqueous electrolyte secondary batteries. Secondary batteries have been practically utilized as electric sources of portable phones, notebook personal computers, etc., and furthermore are tentatively used for medium and large scale uses such as uses for automobiles and uses for storing electric power.
Conventionally known positive electrode active materials are represented by the formula Li2-xMn1-yMyO3-z, wherein M is at least one element selected from the group consisting of Fe, Al, Ni and Co, and x, y and z satisfy 0<x≦0.9, 0≦y≦0.5 and 0<z≦0.9), and Patent Document 1 discloses the positive electrode active materials are used for aqueous electrolyte lithium secondary batteries. Patent Document 1 discloses that Mn in Li2MnO3 is tetravalent and stable, Li ion is hardly eliminated and is electrochemically stable and hence charge-discharge reaction does not generally occur, while the positive electrode active material of the above formula in which oxygen is deficient and Mn is lower than tetravalent, and elimination of Li ion becomes possible until Mn becomes tetravalent. Thus, reversible charge-discharge reaction can occur. Patent Document 1 further discloses a method for producing the positive electrode active material of the above formula which comprises charging in a closed vessel a mixed solution in which a precursor of the positive electrode active material of the above formula is precipitated, and irradiating the vessel with electromagnetic wave of 2.45 GHz with keeping the temperature at 100° C., followed by filtration, water washing and drying.    (Patent Document 1) JP-A-2005-71807 (claims and paragraphs [0018] and [0032])